Ultrasound Standard Imaging
Standard ultrasound imaging consists of an insonification of the medium with a cylindrical wave that focuses on a given point. Using the backscattered echoes of this single insonification, a complete line of the image is computed using a dynamic receive beamforming process. To build a complete image, this procedure is repeated by sending a set of focused waves that scan along a lateral line at given depth (named the focal plane). For each focused wave, a dynamic beamforming is performed and the complete image is obtained line by line. The dynamic beamforming guarantees a uniform focusing in the receive mode, whereas, in the transmit mode the focus is fixed at a given depth. The final image is optimal in the focal plane and in a limited region of the medium corresponding to the focal axial length. However, outside this area which is imposed by diffraction laws, the image quality is rapidly degraded at other depths (in the near and far fields of the focused beam).
To overcome this limitation, a classical solution is to perform multi-focus imaging: different transmit focal depths are used to obtain a homogeneous quality all over the image. Each transmission at a given focal depth enables performing a partial image in the region delimited by the axial focal length. The final image is obtained using a recombination of these partial images corresponding to various depths. An optimal multi-focus image requires typically tens of focal planes. This leads to frame rate limitations, typically <10 frames/second, that are not acceptable for ultrasound imaging. A good compromise between image quality and frame rate is around 4 focal depths images.
Ultrasound Synthetic Imaging
Improvement in image quality can be envisioned by performing synthetic dynamic transmit focalization. Such approach consists in re-synthesizing a dynamic transmit focusing (i.e. as many focal depths as pixel in the image) by beamforming and then combining a set of different insonifications.
Two main implementations can be considered: Synthetic aperture and coherent plane wave compound.
i) Synthetic Aperture
In the synthetic aperture approach, the ultrasonic array is fired element by element, and the complete set of impulse responses between each transmit and receive element is beamformed and recorded, as disclosed for instance in U.S. Pat. No. 6,689,063. It is then possible to post-process these data in order to generate a synthetic image relying on both transmit and receive focusing for each pixel of the image. It has been intensely discussed in the literature whether synthetic imaging could give better images than conventional B-mode images, and how they will be affected by tissue motion and limited signal-to-noise ratio. A fundamental problem in synthetic aperture imaging is the poor signal-to-noise ratio in the images, since a single element is used for emission. This gives a much lower emitted energy compared to using the full aperture in conventional imaging and therefore limits the depth of penetration.
ii) Synthetic Plane Wave Approach
Synthetic plane wave imaging is an approach that solves at least partially the limitations of synthetic aperture imaging. It consists in transmitting plane waves of different angles in the medium, beamforming in receive the backscattered signal then combine the different image to re-synthesize to final image, as disclosed for instance in U.S. Pat. No. 6,551,246. The transmission of a plane wave on the complete array generates a much higher pressure field than in the synthetic aperture approach. Moreover, diffraction and attenuation effects during propagation in soft tissues are significantly lower for an ultrasonic plane wave compared to a single element transmission.
Synthetic dynamic transmit focusing approaches push the boundaries of the classical Image Quality/Frame rate compromise. Optimal image qualities can be obtained at higher frame rates (>10 Hz).
However, the currently known synthetic ultrasound imaging methods which use the plane wave approach still need to be improved in terms of accuracy of the image.